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I switched from the VF router back to my old Asus RT-AC68U and Draytek Vigor 130 around about the start of June. I'm still getting the discount, so somehow I'm still able to trigger it. Whether it's monitored remotely or somehow triggered by my checking the sync speeds I don't know.
*The whole new build estate I'm on has major connectivity problems (and no more lines available) which OpenReach are planning to deal with by re-provisioning the entire estate some time next year. So switching here would not be an option as doing so could even lose us our connection to the cabinet!
I read that you had to apply for the discount via the app, which then connected to the router to determine sync speeds. This could only happy if the VF router was being used.
That said, you could attach the router once per month, request the discount, and then revert to your prefered router.
My VF router hasn't been connected to the wall socket in months! Though that said Openreach now calculate that the absolute maximum speed our cabinet is capable of is just 55Mbps.